Description
Description: Finnish Meteorological Institute, Geophysical Research, PO Box 503, FIN-00101, Helsinki, Finland. A
short event of high-velocity E-region echo observations by
the Pykkvibaer HF radar is analysed to study echo
parameters and the echo relation to the Farley-Buneman plasma
instability. The echoes were detected in several beams aligned
closely to the magnetic L-shell direction. Two echo groups
were identified: one group corresponded to the classical type
1 echoes with velocities close to the nominal ion-acoustic speed
of 400 ms–1
, while the other group had significantly larger
velocities, of the order of 700 ms–1
. The mutual relationship between the echo
power, Doppler velocity, spectral width and
elevation angles for these two groups was
studied. Plotting of echo parameters versus slant range showed
that all ~700 ms–1
echoes originated from larger heights
and distances of 500–700 km, while all ~400
ms–1
echoes came from lower heights and from farther
distances; 700–1000 km. We argue that both
observed groups of echoes occurred due to
the Farley-Buneman plasma instability excited by
strong ( ~70 mVm–1
) and uniformly distributed electric fields.
We show that the echo velocities for the two groups
were different because the echoes were received from different
heights. Such a separation of echo heights occurred due
to the differing amounts of ionospheric refraction at short and
large ranges. Thus, the ionospheric refraction and related altitude modulation
of ionospheric parameters are the most important
factors to consider, when various characteristics of E-region
decametre irregularities are derived from HF radar measurements.